docs: def add_raw_data(self, read_number, data):
          """ Add raw data for a read.
          
          :param read_number: The number of the read the raw data is for.
          :param data: The raw data DAQ values (16 bit integers).
          
          The read must already exist in the file. It must not already
          have raw data.
          """
          self.assert_writeable()
          read_index = self.status.read_number_map[read_number]
          if self.status.read_info[read_index].has_raw_data:
              msg = 'Fast5 file already has raw data for read {} in {}'
              raise KeyError(msg.format(read_number, self.filename))
          self._save_raw(read_number, data)